1. Field of the Invention
The present invention relates to a color conversion table creating device, a color conversion table creating method, and storage medium.
2. Description of Related Art
Generally, in order to create a color conversion LUT (Look Up Table) to convert image data including cyan (C), magenta (M), yellow (Y), and black (K) to CMYK data for output from a printer, it is necessary to print a color patch formed from a combination of CMYK with a printer, and to measure the color patch with a color measuring device to correspond a CMYK value and a color measurement value.
Here, if a color patch formed from a combination of four colors of CMYK is used, the number of measurements becomes large. Therefore, there is known a method to create a three dimensional LUT of CMY→CMY based on the color measurement value of the color patch formed from the combination of CMY and a one dimensional LUT of K→K based on the color measurement value of the tone patch of single color K so that color conversion of CMY component and K component are performed separately.
For example, there is proposed a technique in which the unbalance of gray balance is corrected by using a three dimensional LUT which corrects the CMY component of the image data and a one dimensional LUT which corrects the K component of the image data (see Japanese Patent Application Laid-Open Publication No. 2007-89031).
However, according to the conventional technique, the CMY component and the K component are calculated separately. Therefore, when the color material amount is limited by limiting a total amount of each color component after color conversion, there is a possibility that the CMYK value changes by the color material amount limit, the corresponding relation with the three dimensional LUT used in color conversion does not match and the accuracy of color conversion decreases. When all of the color material (toner, ink, etc.) which can be output is used in a electro-photographic printer, inkjet printer, etc., problems such as fusing defect of toner in an electro-photographic printer and blurring in an inkjet printer, etc. occur. Therefore, normally the total amount of the color material is fixed.
Described below are the problems of the convention processing which separately refers the three dimensional LUT of CMY and the one dimensional LUT of K for the CMYK value to perform color conversion and then limits the toner amount of the CMYK value.
FIG. 14 is an example of conversion when the total CMYK amount is relatively small and FIG. 15 is an example of conversion when the total CMYK amount is large and the total toner amount is adjusted. FIG. 16A is an example of a CMY three dimensional LUT and FIG. 16B is an example of a K one dimensional LUT.
As shown in FIG. 14, when the input CMYK value is (80, 80, 80, 0), first the values are divided between CMY value (80, 80, 80) and the K value (0). For the CMY value (80, 80, 80), the CMY three dimensional LUT shown in FIG. 16A is referred, and the CMY value is converted to (70, 80, 90). For the K value (0), the K one dimensional LUT shown in FIG. 16B is referred, and the K value is converted to (0).
Next, the converted CMY value (70, 80, 90) and the converted K value (0) are combined and it is judged whether there is a toner amount limit on the combined CMYK value (70, 80, 90, 0). Here, when the toner amount limit value (maximum value of total CMYK amount) is 250%, the total CMYK amount is 70+80+90+0=240 and is not more than the toner amount limit value. Therefore, the CMYK value is not limited by the toner amount limit and the final output CMYK value is (70, 80, 90, 0). As described here, when the total CMYK amount is relatively small, the CMYK value does not change by the toner amount limiting and therefore there is no problem.
Alternatively, as shown in FIG. 15, when the input CMYK value is (80, 80, 80, 100), first the values are divided between the CMY value (80, 80, 90) and the K value (100). For the CMY value (80, 80, 80), the CMY three dimensional LUT shown in FIG. 16A is referred, and the CMY value is converted to (70, 80, 90). For the K value (100), the K one dimensional LUT shown in FIG. 16B is referred and the K value is converted to (100).
Next, the converted CMY value (70, 80, 90) and the converted K value (100) are combined and it is judged whether there is a toner amount limit on the combined CMYK value (70, 80, 90, 100). Here, the total CMYK amount is 70+80+90+100=340 and exceeds the toner amount limit value. Therefore, the toner amount is limited and the final output CMYK value is, for example, (40, 50, 60, 100). Here, the CMY value (40, 50, 60) is a value which has no relation with the output value (70, 80, 90) from the referred CMY three dimensional LUT and is a value with no guarantee.